Organocopper compound

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Brief history

The first organocopper compound, the explosive dicopper acetylide Cu₂C₂ was synthesized by Bottger in 1859. Henry Gilman prepared methylcopper in 1936. In 1941 Kharash discovered that reaction of a Grignard reagent with cyclohexenone in presence of Cu(I) resulted in 1,4-addition instead of 1,2-addition. In 1952 Gilman investigated for the first time dialkylcuprates.

Properties

Organocopper compounds are very reactive towards oxygen and water forming copper(I) oxide, tend to be thermally unstable and are generally insoluble in inert solvents. They are therefore difficult to handle and of little practical value. On the other hand organocopper reagents are used very frequently in organic chemistry as alkylating reagents prepared in situ in an inert environment with in general more functional group tolerance than corresponding Grignards or organolithium reagents. The electronegativity of copper is much higher than its next-door neighbour in the group 12 elements, zinc, suggesting less nucleophilicity for carbon.

Copper belongs to the group of coinage metals together with silver and gold and their chemistries have many similarities. The oxidation state can be +1 or +2 and intermediates can have oxidation state +3. Monovalent alkylcopper compounds (R-Cu) form divalent cuprates R₂CuLi with organolithium compounds (R-Li) now known as Gilman reagents. Organocopper compounds can be stabilized with organophosphanes (R₃P).

The cuprates have complex aggregation states in crystalline form and in solution. Lithium dimethylcuprate is a dimer in diethyl ether forming an 8-membered ring with two lithium atoms coordinating between two methyl groups.

The first ever crystal structure was determined in 1972 by Lappert for CuCH₂SiMe₃. This compound is relatively stable because the bulky trimethylsilyl groups provide steric protection. It is a tetramer forming an 8-membered ring with alternating Cu-C bonds. In addition the four copper atoms form a planar Cu₄ ring based on three-center two-electron bonds. The copper to copper bond length is 242 pm compared to 256 pm in bulk copper. In pentamesitylpentacopper a 5-membered copper ring is formed and pentafluorophenylcopper is a tetramer.^[4]

With carbon monoxide copper forms a non-classical metal carbonyl.

Cu(III) intermediates

In many organometallic reactions involving copper, the reaction mechanism invokes a copper intermediate with oxidation state +3. For instance, in reductive elimination processes, Cu(III) is reduced to Cu(I). However Cu(III) compounds are rare in chemistry in general and until recently organocopper(III) species have been elusive. In 2007 the first spectroscopic evidence was obtained for the involvement of Cu(III) in the conjugate addition of the Gilman reagent to an enone ^[5]:

In a so-called rapid-injection NMR experiment at -100°C, the Gilman reagent Me₂CuLi (stabilized by lithium iodide) was introduced to cyclohexenone (1) enabling the detection of the copper - alkene pi complex 2. On subsequent addition of trimethylsilyl cyanide the Cu(III) species 3 is formed (indefinitely stable at that temperature) and on increasing the temperature to -80°C the conjugate addition product 4. According to an accompanying in silico experiments ^[6] the Cu(III) intermediate has a square planar molecular geometry with the cyano group in cis orientation with respect to the cyclohexenyl methine group and anti-parallel to the methine proton. With other ligands than the cyano group this study predicts room temperature stable Cu(III) compounds.

Synthesis

• Copper halides react with organolithium reagents to form the organocopper compound. Phenyl copper is prepared by reaction of phenyllithium with copper(I) bromide in diethyl ether.
• Reaction with a second equivalent of R-Li to R-Cu gives the lithium diorganocopper compound.
• Copper halides also react with Grignard reagents. The compound pentamesitylpentacopper is prepared from mesityl magnesium bromide and copper(I) chloride.
• Copper salts add to terminal alkynes to form copper acetylides
• The copper metallocene (η-cyclopentadienyl triethylphosphine) copper can be prepared by reaction of copper(II) oxide with cyclopentadiene and triethylphosphine in pentane at reflux.

Reactions

Organocopper reactions are classified in a number of reaction types:

• Substitution reactions of cuprates R₂CuLi to alkyl halides R'-X gives the alkylcopper compound R'-Cu, the coupling product R-R and the lithium halide. The reaction mechanism is based on nucleophilic attack, namely oxidative addition of the alkyl halide to Cu(I) elevating it to a planar Cu(III) intermediate followed by reductive elimination. The nucleophilic attack is the rate-determining step. In the case for substitution of iodide, single electron transfer mechanism is proposed.

Many electrophiles will work. The approximate order of reactivity, beginning with the most reactive, is as follows: acid chlorides^[7] > aldehydes > tosylates ~ epoxides > iodides > bromides > chlorides > ketones > esters > nitriles >> alkenes

• Oxidative coupling: coupling of copper acetylides to conjugated alkynes in the Glaser coupling (for example in the synthesis of cyclooctadecanonaene) or to aryl halides in the Castro-Stephens Coupling
• Reductive coupling: coupling reaction of aryl halides with a stoichiometric equivalent of copper metal occurs in the Ullmann reaction. In an example of a present-day cross-coupling reaction called decarboxylative coupling, a catalytic amount of Cu(I) displaces a carboxyl group forming the arylcopper (ArCu) intermediate. Simultaneously, a palladium catalyst converts an aryl bromide to the organopalladium intermediate (Ar'PdBr), and on transmetallation the biaryl is formed from ArPdAr'.^[8][9]

• Redox neutral coupling: the coupling of terminal alkynes with halo-alkynes with a copper(I) salt in the Cadiot-Chodkiewicz coupling
• Thermal coupling of organocopper compounds
• Conjugate additions to enones are done with organocopper (RCu) if a Grignard reagent (such as RMgBr) would instead react in a 1,2-addition.^[10] The mechanism goes through the nucleophilic attack of the alkyl group to form a RCu(III) intermediate.^[11] In the original paper describing this reaction, methylmagnesium bromide is reacted with isophorone with and without 1 mole percent of added copper chloride ^[12]:

Without added salt the main products are alcohol B (42%) from nucleophilic addition to the carbonyl group and diene C (48%) as its dehydration reaction product. With added salt the main product is 1,4-adduct A (82%) with some C (7%).

• Carbocupration is a nucleophilic addition of organocopper reagents (R-Cu) to acetylene or terminal alkynes resulting in an alkenylcopper compound (RC=C-Cu).^[13] The presence of magnesium(I) bromide is generally required. It is a special case of carbometalation.

See also

• Chemistries of carbon with other elements of the periodic table:

References